1. Field of the Invention
The present invention relates to a multilayer printed wiring board, and more particularly to a multilayer printed wiring board capable of dissipating heat that is generated in integrated circuits into the air.
2. Description of the Related Art
In recent years, improvement in the degree of integration of integrated circuits has been accompanied by improvement in the operating speed, but the heat generated by integrated circuits has also increased. Given these circumstances, a technique for dissipating heat for an integrated circuit mounted on a printed wiring board is disclosed in Japanese Patent Laid-open No. 160527/93.
FIG. 1 shows a plan view of an example in which an integrated circuit is mounted on the above-described printed wiring board of the prior art, and FIG. 2 is a sectional view of FIG. 1 taken along line A-Axe2x80x2. In this example of the prior art, printed wiring board 82 includes: planar conductor 88 provided on surface layer 85, on which integrated circuits 81 are mounted, so as to confront integrated circuit 81; planar conductor 84 provided on rear layer 86 on the opposite side of printed wiring board 82 for dissipating heat; and a plurality of through holes for connecting planar conductor 88 and planar conductor 84. Integrated circuit 81 is mounted on printed wiring board 82, and integrated circuit 81 and printed wiring board 82 are connected together by adhesive 90. Integrated circuit 81 is mounted on multilayer printed wiring board 82 with its terminals 95 fixed and connected to pads 93 by solder 98. Gap 92 between integrated circuit 81 and multilayer printed wiring board 82 is produced by stand-off 96 between terminals 95 and main body 94 of integrated circuit 81. The heat generated by integrated circuit 81 is conducted to planar conductor 88 by way of adhesive 90, further conducted to planar conductor 84 by through holes 83, and is then dissipated by discharging it into the air.
Japanese Patent Laid-open No. 5747/94 also describes a technique by which the heat of an integrated circuit that is mounted on a printed wiring board is conducted by way of a thermally conductive resin to a conductor pattern of inner and outer layers
However, the above-described techniques of the prior art suffer from the following disadvantages.
First, in the prior-art example shown in FIG. 1 and FIG. 2, since the planar conductors of the surface layer and rear surface layer are connected by through holes, the planar conductor of the rear surface layer must be positioned on the rear surface corresponding to the position at which the integrated circuit is mounted, thereby imposing limits on the signal wiring of the printed wiring board.
In the prior-art example described in Japanese Patent Laid-open No. 5747/94 the arrangement of an outer-layer conductor pattern has an influence on signal wiring and mounting components around the periphery of the integrated circuit. On the other hand, giving priority to the signal wiring and mounting components around the periphery of the integrated circuit, prevents the arrangement of an adequate conductor pattern.
Furthermore, adequate heat dissipation cannot be achieved because an interlayer insulator, which has low thermal conductivity when dissipating heat into the air, is present between the outer surfaces of the printed wiring board.
It is therefore an object of the present invention to provide a multilayer printed wiring board in which no limitation is placed on signal wiring of the printed wiring board.
To achieve the aforementioned object, the multilayer printed wiring board according to the present invention comprises: at least one injection hole for injecting a thermally conductive filler between an integrated circuit and a planar conductor; filled-state inspection holes for verifying whether the thermally conductive filler that has been injected from the injection hole has spread over the necessary area; a heat dissipating planar conductor that is arranged on an outer layer that is at a location other than the location at which the integrated circuit is mounted and that is provided for dissipating heat of an inner-layer planar conductor into the air; and through holes for thermally connecting the heat dissipating planar conductor and the inner-layer planar conductor.
Heat that is conducted from an integrated circuit can be conducted to a location other than the position at which the integrated circuit is mounted, by using a planar power supply conductor or a planar ground conductor that is already present in an inner layer of the multilayer printed wiring board, and a heat dissipating planar conductor of the size necessary for dissipating heat into the air can dissipate heat into the air at one position or be divided among a plurality of positions. Accordingly, even in cases in which an integrated circuit generates a great amount of heat, there is no need to provide an unreasonably large planar conductor solely for dissipating heat on the rear surface of the multilayer printed wiring board at the position at which the integrated circuits are mounted, and as a result, any influence on the signal wiring and component mounting in the vicinity of the integrated circuit can be suppressed to a minimum.
In addition, the provision of an injection hole for the thermally conductive filler enables injection of the thermally conductive filler into the gap between the integrated circuit and the multilayer printed wiring board from the rear surface of the multilayer printed wiring board after the integrated circuit has been mounted on the multilayer printed wiring board.
Further, the provision of holes for verifying the filled state of the thermally conductive filler can prevent underfilling or overfilling because it allows verification during injection that the injected thermally conductive filler has spread to the necessary areas.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.